The present invention relates to a gain control circuit for controlling a signal level, and more particularly relates to a variable gain control circuit for regulating the level of a signal supplied to a load by controlling the gain of an amplifier.
In a transmission line for transmitting a low frequency signal or a high frequency signal, and an amplifier for amplifying such signals, it is necessary to control the amplitude of a signal flowing through the transmission line or the amplifier, as the case may be, in order to attain the proper level of the output signal thereof.
A variable attenuator is commonly used as a conventional device for controlling the amplitude of a signal. A low frequency signal, a high frequency signal or the like is temporarily amplified to a sufficiently high level and then attenuated by a variable attenuator, thereby producing output signal of proper level. The variable attenuator comprises a variable resistor in most cases. The variable resistor in turn includes an insulating substrate, a resistor member disposed on the insulating substrate and a slider slidably in contact with the surface of the resistor member. Thus, a division of the signal applied to the resistor member is obtained. The slider is adapted to slide mechanically along the surface of the resistor member.
In controlling the signal level by use of this resistor member, the slider of the variable resistor is required to be slided mechanically. Therefore, in the case where the remote control of the mechanical operation is carried out, it is necessary to provide a driver for moving the slider of the variable resistor, so that the gain control system becomes complex.
On the other hand, a variable gain amplifier has hitherto been proposed in which the amplitude of a signal can be increased or decreased continuously without any mechanical operation. This variable gain amplifier includes two amplifying transistors of the same conductivity type with the emitters thereof connected to each other. The collector-emitter path of a voltage/current conversion transistor is connected between the junction point of the amplifying transistors and ground so that a signal is applied to the base of the voltage/current conversion transistor. The base of one of the two amplifying transistors is supplied with a fixed bias voltage, while the base of the other amplifying transistor is connected to a variable voltage bias source, the voltage of which may be changed, thus making it possible to take out an output signal from the collector of the other amplifying transistor. The two amplifying transistors make up a differential amplifier. By changing the base bias voltage of the other transistor, the collector current thereof is increased or decreased as desired, so that the collector current of one amplifying transistor is decreased or increased in accordance with the increase or decrease in the collector current of the other amplifying transistor, respectively, with the result that a signal having an amplitude proportional to the collector current of one amplifying transistor is produced from the collector of the other amplifying transistor.
The output signal of this variable gain amplifier undesirably contains a distortion. As described in detail later, each of the emitter, base and collector layers of a transistor has a bulk resistance relating to semiconductors making up the transistor. Further, a bonding resistance exists between the emitter electrode, base electrode or collector electrode and the respective terminals connected with each of the electrodes. Thus, the transistor presents such a condition that a resistor of a value equal to the sum of the bulk resistance and the bonding resistance would be connected to each of the emitter, base and collector of the transistor. Accordingly, each of the above-mentioned two amplifying transistors of the variable gain amplifier has a bulk resistance and a bonding resistance connected to the emitter, base and collector thereof. Of these bulk resistance and bonding resistance, those connected to the emitter cause a distortion in the output signal. In the collector-emitter conduction path of the two amplifying transistors, a DC current determined by the base-emitter bias voltage and an AC signal current supplied from the voltage/current conversion transistor flow through the bulk resistor and the bonding resistor, so that voltages are generated across the bulk resistor and the bonding resistor for the emitters of the transistors by the DC current and the AC signal current. Of these voltages, the AC voltage generated by the AC signal current changes the base-emitter bias voltage of the respective amplifying transistors, resulting in a distortion of the output signal. The low-frequency wave amplifier for amplifying a music signal recorded in a disc record is especially required to produce an output signal which is low in distortion. The variable gain amplifier which produces the distortion in the output signal cannot be used as the low-frequency amplifier.